In the world I'm making I want to have very large creatures that will later be used in organic architecture as housing for the human equivalent in this world. In order to make this world realistic I am only going to be pushing the creatures to the upper limit of the Square-Cube Law. This has been discussed at length on this site and there are quite a few examples of the size limits of different body types and activity levels.
In order to have the city sized creatures I want, I plan on shrinking my humanoids to the smallest size that can still be mistaken for full size. Let me clarify a little better. How small can I make my humanoids before we start getting weird physics and biology? I don't want them to be carrying water in their hands and throwing it like water balloons and they need to have a very similar biology to full sized humans (no open circulatory system, they have to be warm blooded, and they can't be able to jump 100x their body length). I know someone already asked about microscopic humans but I want to find the sweet spot between small size and normal physics (yes I know this question is about use of fire but I couldn't find the one that asked about anatomy).
You can ignore comparisons to other plants and animals. I will be adjusting their size as well and most likely creatures like mice will be replaced with relatively larger bugs.